There are many situations in industry today requiring a chemically inert surface which has substantial wear resistance. Alumina (aluminum oxide) is well known for its chemical inertness in many environments. However, alumina is limited in its applicability as a coating material on most metallic substrates at elevated temperatures. For example, detonation gun and plasma sprayed coatings of pure alumina have been successfully used in the food processing and textile industries at ambient temperatures where advantage can be taken of their high wear and corrosion resistance. However, when heated to moderately elevated temperatures, e.g., about 110.degree. to 440.degree. C., alumina coatings usually spall from most metal substrates, for example, aluminum, carbon steel, stainless steels, or titanium. Similar problems occur with other refractory oxides when used as coatings such as zirconia and chromia. This, in part, is due to the severe mismatch in thermal expansion between refractory oxides and these metals or alloys. Consequently, any advantage that might otherwise be gained from these refractory oxides coatings, namely, wear and corrosion resistance, is lost.
The tendency to spalling of alumina coatings is exacerbated by the fact that alumina upon thermal cycling may go between the alpha and gamma crystallographic phases with disruptive volumetric changes. These changes, like the mismatches in thermal expansion, can cause spalling.
A unique family of coatings containing alumina with a cobalt base matrix is disclosed in U.S. Pat. No. 4,124,737. These coatings, which contain up to 50 volume % alumina or other oxides, possess unique wear, thermal shock and impact resistance.
Unfortunately, this family of coatings does not exhibit adequate chemical inertness in all environments. For example, in those situations where metals or oxidized metals come into contact with the coating surface at elevated temperatures, e.g., above about 500.degree. C., reactions may occur between the matrix of the coatings and the contacting metal or metal oxides. These reactions result in transient or permanent bonding, galling wear and/or pick-up on the surface of the coating.
U.S. Pat. No. 3,436,511 discloses coatings applied by arc processes which coatings consist essentially of from 20 to 40 weight percent of at least a metal taken from the group consisting of chromium, nickel and aluminum with the remainder being at least one oxide taken from the class consisting of chromia, alumina and titania. These coatings have a nodular finish and are particularly suggested for use in the textile industry to provide a wear resistant, low friction surface over which threads, filaments, yarns, etc., can be passed and for use in the aircraft industry. The Table and supporting text indicates that a chromia chromium mix containing 60 percent chromia and 40 percent chrome (approximately 75 volume percent chromia) had good wear properties above about 1000.degree. F. but poor at room temperature. A chromia-chromium alloy containing 80 percent chromia and 20 percent chromium (approximately 90 volume percent chromia) demonstrated a higher wear rate at elevated temperatures but was better than the above mentioned chromia-chromium mix. An alumina-aluminum (approximately 75 volume percent alumina) and alumina-nickel (approximately 75 volume percent alumina) were also exemplified.
U.S. Pat. No. 3,864,093 discloses high temperature, wear resistant coatings having at least one metal oxide dispersed in a metal alloy matrix comprising (a) at least 40 percent based on the weight of the alloy of at least one of iron, cobalt and nickel and (b) between 10 and 40 percent based on the weight of the alloy of at least one of silicon, aluminum and chromium. The patent further discloses that other components such as tungsten, molybdenum, vanadium, manganese, carbon, rhenium, yttrium, lanthanum, boron, niobium, titanium, tantalum and zirconium may be present. The metal oxide is present in an amount from about 2 to 50 volume percent and may be aluminum oxide, chromium oxide, zirconium oxide as well as other metal oxides.
U.S. Pat. No. 3,597,241 discloses metallo-ceramic compositions having at least three components: 15 to 60 percent chromium, 10 to 50 percent nickel and 10 to 40 percent ceramic oxide such as alumina and stabilized zirconia. The patent discloses that it is difficult to anchor such ceramic oxides to substrates due to the thermal stresses that cause separation. The patent discloses the use of a first metallo-ceramic layer and then a ceramic overlay. A metallo-ceramic layer containing 40 percent alumina (approximately 60 volume percent), 30 percent chromium and 30 percent nickel was used in Example 5 as an intermediate layer with an alumina overlay.
The Derwent Abstract for Japanese patent application Kokai No. 156773/1981 reports a radiator body for infrared cooking applications which has been coated with a sprayed layer of metal oxide such as alumina, zirconia and chromia with the inclusion of metals in the coating being less than 10 percent. This would be expected to provide at least 95 volume percent metal oxide in the coating. The coatings are said to have excellent heat resistance, corrosion resistance, durability and performance.
U.S. Pat. No. 4,180,622 discloses wear resistant coatings for, e.g., disc brakes, made from aluminum. The coatings disclosed are cermets containing between 10 to 40 weight percent aluminum and 90 to 60 weight percent ceramic oxide such as alumina, chromia, and mixtures of alumina with one of titania, zirconia and chromia. The patent states that the amount of metal should be sufficient to provide ductility insensitivity and insensitivity to shock.
British Patent Specification No. 2,117,415A discloses a multiple layer coating having a nickel-chromium alloy first layer, a second layer having a mixture of the same alloy of the first layer and 30 to 70 weight percent aluminum oxide. A final layer alumina may be applied. The multilayer coating is said to provide a heat resistant coating having improved peeling and corrosion resistance. At page 3, lines 3 to 5, the patent discloses that the final alumina layer is required to obtain desired corrosion resistance.
Although this British Patent Specification No. broadly suggests the use of 30 to 70 weight percent aluminum oxide in the second layer, only a 30 weight percent alumina-containing layer (about 45 volume percent) is exemplified as being within the scope of the invention. Test No. 6, for instance, is comparative and uses as the second layer 20 parts of alloy to 80 parts of alumina (approximately 90 volume percent alumina). As can be seen from FIG. 2, significant weight loss of this coating occurs with thermal cycling which is indicative of spalling. Not only does the British Patent Specification demonstrate the cracking and spalling problems with high alumina volume fraction coatings, but it further indicates that even with lower alumina volume fraction coatings, alloys of nickel and chromium are insufficient to prevent spalling. See, for example, tests number 7 and 8 in which the second layer has 30 parts of alumina (approximately 45 volume percent) to 70 parts of a 50:50 alloy of nickel and chromium. Apparently because of the base coating layer, significant weight loss upon thermal cycling occurred which is indicative of spalling. Accordingly, the applicant in the British Patent Specification requires that, even with the lower volume fraction alumina coatings, the metal matrix be the same composition as the base coat matrix.
The British Patent Specification therefore places in question the broad assertions in prior disclosures regarding the ability to prevent spalling in high volume fraction refractory oxide coatings, in single layer coatings or even coatings having a base overlay with the alumina containing coating placed on the base overlay.
A number of workers in the field have proposed multilayer coatings in which the composition of metal oxide was gradually increased from essentially zero at the interface with the substrate to virtually 100 percent metal oxide at the surface. See, for instance, U.S. Pat. Nos., 3,091,548; 3,719,519 and 3,977,660 and Karpinos, et al., "Improving the Adhesion of Plasma-Sprayed Coatings to Articles", Poroshkovaya Metallurgiya, No. 3 (111), pp. 106-107, March, 1972 These workers tend to support the observations reported in the British Patent Specification that the solution to the problem of thermal cycle resistant, high volume fraction metal oxide coatings requires the use of base coating(s) having particularly matched bonding and thermal expansion properties.